Biomedical subjects

M Burrows

Publications and source records attributed to M Burrows.

At least 55 records · Page 3Linked to original sources

PubMed1992

Reliability and effectiveness of transmission from exteroceptive sensory neurons to spiking local interneurons in the locust.

Mechanosensory information from exteroceptive hairs on the legs of a locust is first processed in a segmental ganglion by a midline population of spiking local interneurons for use in adjustments of posture and locomotion. Each interneuron receives excitatory inputs from a characteristic array of these receptors so that the surface of a leg is mapped onto the whole population of interneurons as a series of overlapping receptive fields. The properties of this first synaptic connection, and the contributions of individual afferents forming the receptive fields of the interneurons are examined. The gain of the excitatory synaptic connection between the hair afferents and the interneurons is often high, so that a single afferent spike can lead directly to a spike in the interneuron. Repetitive spikes in a hair afferent evoke EPSPs in an interneuron that decline in amplitude but that may summate. The first EPSP in any sequence is always the largest. The high frequencies of afferent spikes that are evoked by a normal deflection of a hair saturate the synaptic connection so that the amplitude of depolarization is no greater than to a single spike. The EPSPs from two hairs in a receptive field can summate but lead to no heterosynaptic facilitation. High-frequency bursts of spikes in one afferent can reduce the postsynaptic effect of another afferent. The amplitude of the EPSPs and the gain of the synaptic connections differ markedly between the hairs that comprise the receptive field of an interneuron. There are gradients of effectiveness, generally according to the axes of the leg, with one group of adjacent hairs producing the largest-amplitude EPSPs and having the highest gains. Individual hairs may contribute to the receptive field of more than one interneuron, and the gain of these connections may differ. The complexity of a receptive field is further accentuated by the specificity of connections made by the different physiological types of hair receptors. High-threshold hairs may make synaptic connections with an interneuron, but adjacent low-threshold hairs may not. This organization of the receptive fields means that the interneurons are sensitive to certain inputs and can reliably pass on a signal from one hair. It also implies that greater weighting is given to inputs from certain regions.

Animals↗

PubMed1991

Pharmacokinetics and fate of 3H-trospectomycin sulphate, a novel aminocyclitol antibiotic, in male and female rats.

1. The pharmacokinetics and fate of 3H-trospectomycin sulphate, a novel aminocyclitol antibiotic, were examined in male and female rats after intramuscular (i.m.), intravenous (i.v.) and subcutaneous (s.c.) dosing. 2. Total radioactivity levels in plasma were associated with unchanged trospectomycin. Two radioactive components were found in urine, one was indistinguishable from trospectomycin and the other was probably a degradation product formed after excretion or during storage rather than a metabolite. 3. The disappearance of drug from plasma followed a biphasic pattern with half lives of 0.3-0.4 h and 45-80 h and a large distribution volume, which indicated some retention of drug by tissues. Clearance rates were within the normal range for glomerular filtration rate, which indicated that the primary process of elimination is filtration of unchanged drug. 4. Excretion was initially rapid (greater than 40% by 4 h) and mainly into urine (faecal excretion greater than 20%). Urinary excretion was significantly larger in males than females but faecal excretion was significantly smaller, so that there was no significant difference in total excretion. 5. The bioavailability following s.c. dosing was only approximately 75% but there were few other biologically significant differences between the routes of administration. Absorption following i.m. and s.c. dosing was rapid. 6. Clearance rate and volume of distribution were higher in males than females. Over the dose range 50-200 mg/kg the pharmacokinetics appeared to be mostly linear.

Animals↗

PubMed1990

Input and output connections of an anteromedial group of spiking local interneurons in the metathoracic ganglion of the locust.

Intracellular recordings were made from an anteromedial group of spiking local interneurons in the metathoracic ganglion of the locust to determine the input connections that shape their mechanoreceptive fields on a hindleg and the output connections that define their effects on hindleg motor neurons. The receptive fields of these interneurons may contain excitatory and inhibitory regions. An excitatory region on the ipsilateral hind leg is formed by direct excitatory connections of the afferents from exteroceptors. Afferent spikes consistently evoke EPSPs in interneurons with a central synaptic latency of 1.2-1.5 msec. The connections appear to be direct and chemically mediated. An inhibitory region of a receptive field is not formed by direct afferent connections. Instead, spiking local interneurons of a midline group, which also receive direct excitatory inputs from the afferents, make direct inhibitory connections with anteromedial interneurons. Spikes in particular midline interneurons consistently evoke IPSPs in anteromedial interneurons with a central synaptic latency of about 1.0 msec. The connections appear to be direct and chemically mediated. By contrast, spikes in anteromedial interneurons could not be linked to any synaptic potentials in midline interneurons, so that direct connections are unidirectional and inhibitory. The large inhibitory regions to the receptive fields of anteromedial interneurons result from convergent inhibitory inputs of several midline interneurons. Each midline interneuron may make inhibitory output connections with several anteromedial interneurons, but there is a high degree of specificity in these divergent connections. Anteromedial interneurons make excitatory output connections with specific hind leg motor neurons. EPSPs in motor neurons follow spikes in the interneurons with short and consistent synaptic latencies of about 1.0 msec. The connections appear to be direct and chemically mediated. Two or more anteromedial interneurons with overlapping receptive fields may converge onto one motor neuron, and, in turn, one interneuron may make divergent connections with several motor neurons of the same pool. This pattern of connections suggests that these interneurons are essential elements in the local circuitry responsible for the expression of local reflexes of a leg. They provide an excitatory drive to the motor neurons in parallel to that from nonspiking local interneurons when particular arrays of exteroceptors are stimulated.

Action Potentials↗

PubMed1989

Effects of temperature on a central synapse between identified motor neurons in the locust.

Changing the temperature from 10-40 degrees C modifies the transmission at an established monosynaptic connection between the fast extensor tibiae (FETi) and flexor tibiae motor neurons in the metathoracic ganglion of the locust Schistocerca gregaria (Forskål). Striking changes occur to the shape of the spikes, to membrane resistance, to the synaptic delay, and to the evoked synaptic potentials. In the presynaptic FETi motor neuron, raising the temperature reduces the amplitude of an antidromic spike recorded in the soma by a factor of 10 (40 mV to 4 mV), reduces the time taken to reach peak amplitude by 5 (3.5 to 0.7 ms) and decreases the duration at half maximum amplitude by 0.5. The conduction velocity of the spike in the axon is increased by 50% from 10 degrees C to 40 degrees C. Orthodromic spikes are affected by temperature in a similar way to the antidromic spikes. The membrane resistance of both pre- and postsynaptic motor neurons falls as the temperature is raised. The membrane resistance of FETi falls by a factor of 4 (about 4 M omega at 10 degrees C to 1 M omega at 40 degrees C). A contributory component to this fall could be the increase in the frequency of synaptic potentials generated as a result of inputs from other neurons. No temperature dependence could be demonstrated on the voltage threshold relative to resting potential for evoking orthodromic spikes, but because the resistance changes, the current needed to achieve this voltage must be increased at higher temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1989

Processing of mechanosensory signals in local reflex pathways of the locust.

The processing of mechanosensory signals responsible for the reflex adjustment of the posture or movement of the legs of the locust is described in terms of the actions and connections of identified neurones. Signals can be followed from the major classes of exteroceptors of a leg, through their various integrative stages in the central nervous system to their emergence as specific patterns in known motor neurones. Particular emphasis is placed on the integrative roles of two classes of local interneurones. The spiking local interneurones map the leg as a series of overlapping receptive fields and reverse the sign of the afferent input. The nonspiking local interneurones control the output of the motor neurones by the graded release of chemical transmitter and can adjust the gain of a local reflex depending on the position and movements of the joints of that leg. The reflex movements of one leg must not impair the stability of the animal and must therefore be influenced by events at the other legs. Populations of intersegmental interneurones convey sensory information from one segment to another to ensure such coordination. These interneurones do not produce stereotyped intersegmental reflexes but, instead, alter the performance of a local reflex in a distant leg by making synaptic connections with nonspiking local interneurones. These connections change the effectiveness of the outputs to the motor neurones and consequently the local reflex. The local interneurones therefore play a crucial role both in the production of local reflexes and in the integration of these actions with the movements of the other legs.

Afferent Pathways↗

PubMed1989

Distribution of intersegmental inputs to nonspiking local interneurons and motor neurons in the locust.

Intersegmental interneurons in a mesothoracic population that receive inputs from extero- and proprioceptors on a middle leg of a locust (Laurent, 1987a) make direct synaptic connections with nonspiking local interneurons and motor neurons controlling the movements of the ipsilateral hindleg. Of 25 direct connections that were established, 80% are excitatory, and 60% are made with nonspiking interneurons. Two or more intersegmental interneurons with outputs of either polarity and with different or overlapping receptive fields can converge onto one metathoracic nonspiking interneuron. Direct connections were found with motor neurons of the coxotrochanteral, femorotibial, and tibiotarsal joints of the hindleg. An intersegmental interneuron can make divergent connections with several motor neurons of the same pool or with a motor neuron and a nonspiking interneuron presynaptic to it. By virtue of the lateral inhibitory connections between nonspiking interneurons controlling different pools of motor neurons (Burrows, 1979), an intersegmental interneuron can excite directly one pool and disynaptically inhibit the antagonistic pool. Metathoracic nonspiking interneurons also receive inputs from afferents of mechanoreceptors on a hindleg. They thus have 2 receptive fields: one on the hindleg whose motor neurons they control and one on the ipsilateral middle leg, provided by inputs from the mesothoracic intersegmental interneurons. Nonspiking local interneurons are thus a point of convergence of local and intersegmental pathways. The effectiveness of the intersegmental pathways can thus be modified by local inputs to the nonspiking interneurons.

Animals↗

PubMed1989

Intersegmental interneurons can control the gain of reflexes in adjacent segments of the locust by their action on nonspiking local interneurons.

The gain of local reflexes of one leg of a locust can be altered by mechanosensory inputs generated by movements of or tactile inputs to an adjacent leg. Touching the mesothoracic tarsus, for example, increases the number of spikes that are produced by the metathoracic slow extensor tibiae motor neuron and enhances the depolarization of flexor tibiae motor neuron in response to imposed movements of the chordotonal organ in the ipsilateral hind femur. The sensory information from the middle leg is conveyed directly to nonspiking interneurons and motor neurons controlling the movements of the hindleg by a population of mesothoracic intersegmental interneurons (Laurent and Burrows, 1989). The metathoracic nonspiking interneurons receive direct inputs from receptors on a hindleg and are, therefore, a point of convergence for local and intersegmental inputs. We examine here the role of the connections between mesothoracic intersegmental interneurons and metathoracic nonspiking interneurons in controlling metathoracic local reflexes. The amplitude of synaptic potentials evoked in leg motor neurons by the stimulation of local afferents can be modulated by altering the membrane potential of an interposed nonspiking interneuron with current injected through an intracellular electrode. These imposed voltage changes mimic a mesothoracic input and show that the state of a nonspiking local interneuron is a determining factor in the expression of a local reflex. Inputs from mesothoracic intersegmental interneurons may cause large changes in the input conductance of nonspiking interneurons that can shunt a local afferent input. In some nonspiking interneurons, synaptic potentials caused by mesothoracic interneurons can be recorded, but no underlying conductance change can be detected at the recording site. Similarly, a particular nonspiking interneuron may receive synaptic inputs when two distinct regions of a middle leg are touched, but only one of these intersegmental inputs may be effective in reducing the amplitude of a synaptic potential caused by afferents from the hindleg. These results suggest that nonspiking local interneurons may be compartmentalized, with synaptic inputs and their associated conductance changes restricted to particular branches. In this way, an individual nonspiking neuron could contribute simultaneously to several local circuits. The inputs from different intersegmental interneurons could then modulate these pathways independently.

Animals↗

PubMed1988

A population of ascending intersegmental interneurones in the locust with mechanosensory inputs from a hind leg.

A population of some 35 intersegmental interneurones with somata in the metathoracic ganglion has been characterized by intracellular recording and staining. These interneurones integrate signals from extero- and proprioceptors on a hind leg. The somata are clustered in an anterior and lateral region of the dorsal cortex, and the axons project to more anterior ganglia in either the ipsilateral or contralateral connectives. Some of these interneurones are excited by afferents from a proprioceptor at the femorotibial joint, the femoral chordotonal organ. An afferent spike evokes a chemically mediated EPSP in an interneurone with a latency and consistency that suggest that the connection is direct. An individual interneurone codes particular features of the movement about the femorotibial joint, responding to flexion, extension, or both directions of movement with either phasic or tonic responses. These interneurones have an extensive field of fine branches ipsilateral to the hind leg from which they receive input. These branches are in lateral and intermediate regions of neuropil to which the afferents of the chordotonal organ also project. Axonal branches, from either an ipsilateral and contralateral axon, are sparse and varicose and occur in dorsal neuropil. Other interneurones are excited by afferents from exteroceptive hairs (trichoid sensilla). An individual interneurone is excited by a particular array of hairs on specific regions of a hind leg. The connections between the afferents and the interneurones appear direct. These interneurones have a dense and compact array of fine branches ipsilateral to the hind leg from which they receive input. These branches are in the most ventral region of neuropil, to which the hair afferents also project. Branches from the ipsilateral axons are sparse and varicose and occur in more dorsal neuropil. The interneurones can thus provide the more anterior ganglia with precise information about the movement of a joint in a hind leg and of the location of an exteroceptive stimulus. This information would be of importance in ensuring the correct co-ordination of the legs during walking.

Animals↗

PubMed1988

Distribution and morphology of synapses on nonspiking local interneurones in the thoracic nervous system of the locust.

The structure and distribution of synapses on nonspiking local interneurones in the metathoracic ganglion of the locust was revealed by electron microscopy following intracellular injection of horseradish peroxidase (HRP). Before staining, each interneurone was characterized physiologically as nonspiking and its output effects on motor neurones innervating muscles in a hindleg were investigated. Three nonspiking interneurones of different morphologies, each typical of a previously described population, were selected for detailed study. The first has a dorsal soma and ipsilateral neuropilar branches, the second a ventral soma and ipsilateral branches, and the third a ventral soma and contralateral branches. The somata have few trophospongial invaginations, and most of their volume is occupied by the nucleus. The initial parts of the primary neurites are either wrapped in glia or isolated in tracts from the neuropile and thus do not participate in synaptic interactions. Some of the larger secondary neurites are also wrapped in glia, but others both make and receive synaptic contacts. Output synapses have an array of some 500-1,600 round, agranular vesicles (diameter 47.0 +/- 5.7 nm; mean +/- S.D., n = 97) associated with a bar-shaped presynaptic density up to 0.3 micron long. Two postsynaptic processes, whose diameter can vary greatly, are usually associated with each presynaptic density. Processes making input synapses onto nonspiking local interneurones typically contain round, agranular vesicles and often make several contacts within a few microns. Serial reconstructions from one of the interneurones revealed input and output synapses intermingled on the larger processes with outputs dominating by a factor of 3:1, whereas on some of the thinner processes only input synapses are present. In the other two interneurones, however, both input and output synapses are present on the fine branches. No feature of the structure or distribution of synapses observed here on the nonspiking local interneurones distinguishes them from spiking neurones in the same ganglia.

Animals↗

PubMed1988

Responses of spiking local interneurones in the locust to proprioceptive signals from the femoral chordotonal organ.

The responses of spiking local interneurones of a ventral midline population in the metathoracic ganglion of the locust, Schistocerca gregaria, to controlled movements of a proprioceptor, the femoral chordotonal organ (FCO) in a hindleg, were revealed by intracellular recording. Afferents from the FCO which signal specific features of the movement or angle of the femoro-tibial joint, can make direct excitatory synapses with particular interneurones in this population (Burrows 1987a). Some interneurones in this population are excited only by flexion, some only by extension, but others by both flexion and extension movements of the femoro-tibial joint. Interneurones excited by one direction of movement may be either unaffected, or inhibited by the opposite movement. The balance between excitation and inhibition is determined by the range over which the movement occurs, and can increase the accuracy of a representation of a movement. The response of some interneurones has tonic components, so that the angle of the joint over a certain range is represented in the frequency of their spikes. Different interneurones respond within different ranges of femoro-tibial angles so that information about the position of the joint is fractionated amongst several members of the population. These interneurones respond to repetitive movements, similar to those used by the locust during walking, with bursts of spikes whose number and frequency are determined by the repetition rate and amplitude of the movement. A brief movement of the FCO may induce effects which persist for many seconds and outlast the changed pattern of afferent spikes. The sign of such an effect depends upon the preceding history of stimulation. Other interneurones respond only to movement so that their response is more phasic. The velocities to which they respond fall within the range of those generated by twitches of the flexor and extensor tibiae muscles and the movements of the tibia during locomotion. Some interneurones respond only to a specific range of velocities because they are inhibited by all other movements. Some interneurones respond to repetitive movements with reliable bursts of spikes, whilst in others the frequency of spikes may be raised but may contain no cyclical information.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1988

Proprioceptive inputs to nonspiking local interneurons contribute to local reflexes of a locust hindleg.

Local reflexes of a leg of the locust Schistocerca gregaria (Forskal) can be elicited by selective stimulation of a proprioceptor (the femoral chordotonal organ) at the femorotibial joint. Motor neurons are either excited or inhibited, so that a coordinated reflex response of a leg results. At the same time, some nonspiking local interneurons are either excited or inhibited by the inputs from these proprioceptive afferents. Altering the membrane potential of an individual, nonspiking interneuron can either increase or decrease the response of the participating motor neurons to the proprioceptive stimulus and thereby alter the gain of the reflex. To determine the pathways, and to understand the role of the nonspiking interneurons in mediating these reflex effects, recordings were made simultaneously from these interneurons and afferent neurons. The excitation of a particular nonspiking local interneuron is produced monosynaptically by the afferent neurons. Chemically mediated EPSPs consistently follow sensory spikes with a latency that is the same as that for the known parallel, direct connections made by these sensory neurons with motor neurons (Burrows, 1987a). The chordotonal afferents and the branches of the local interneurons project to the same regions of neuropil. In contrast, the simplest inhibitory pathway is disynaptic, involving spiking local interneurons. The afferents make direct excitatory connections with some of these spiking interneurons, which then make direct inhibitory connections with a nonspiking interneuron. Interactions between the local interneurons add to the complexity of the pathways.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1987

A strand receptor with a central cell body synapses upon spiking local interneurones in the locust.

Movements of the femoro-tibial joint of a locust hind leg are monitored by three classes of proprioceptors; a chordotonal organ (Usherwood et al. 1968), multipolar joint receptors (Coillot and Boistel 1968) and a strand receptor innervated by a single afferent with a central cell body (Bräunig 1985). All three classes are excited by imposed or voluntary extension of the tibia. The strand receptor (fe-tiSR) spikes tonically and at a frequency dependent upon the position of the joint whilst the multipolar joint receptors give overlapping information but for a more restricted range. The afferent from the strand receptor makes an excitatory connection with a spiking local interneurone in the midline group of the metathoracic ganglion. The central latency and consistency with which the EPSP follows each sensory spike suggests that the connection is direct. This interneurone also receives convergent inputs from neurones in the chordotonal organ, but not from multipolar joint receptors. Neither the strand receptor nor the multipolar joint receptors apparently synapse upon leg motor neurones that we have tested, in contrast to receptors in the chordotonal organ.

Animals↗

PubMed1987

Does the rat have an empty stomach after an overnight fast?

Variable amounts of food have been observed in the stomachs of male rats following an overnight fast. The effects of diet and diet type on the amount of residual food left in the stomach and on fat deposition and liver weight in the male rat were investigated. The implications of these results on metabolism and pharmacokinetic studies are discussed.

Animals↗

PubMed1987

Immunocytochemical and pharmacological evidence for GABAergic spiking local interneurons in the locust.

In the locust thoracic nervous system, spiking local interneurons within a ventral midline population are stained by a polyclonal antibody raised against GABA. Their cell bodies, their primary neurites in ventral commissure II, and their prominent neurites in the perpendicular tract linking ventral and dorsal fields of fine branches are all stained. Individual interneurons in this population were labeled with Lucifer yellow after their receptive fields had been determined physiologically. Alternate sections were stained with the antibody. Some, but not all, of the spiking local interneurons labeled with Lucifer yellow are also stained with the antibody. Apart from having somata that lie more posteriorly within the midline population, the antibody-stained interneurons cannot be distinguished on morphological or physiological grounds from those that are unstained. The way in which this cytochemical heterogeneity within an otherwise homogenous population might arise during development is discussed. A second group of spiking local interneurons with similar sensory input to that of the ventral midline population, but with cell bodies in the anterior lateral region of the ganglion and primary neurites in ventral commissure I, are not stained. Interneurons in the midline group receive direct inputs from sensory neurons and some directly inhibit particular leg motor neurons that mediate interjoint and tactile reflexes. Picrotoxin reversibly blocks the inhibition of the motor neurons and therefore abolishes these reflexes. Immunocytochemical and pharmacological experiments thus suggest that inhibition by some spiking local interneurons may be mediated by GABA.

Action Potentials↗

PubMed1987

Parallel processing of proprioceptive signals by spiking local interneurons and motor neurons in the locust.

The connections made by afferents from a proprioceptor at the femorotibial joint in a hind leg of a locust, the femoral chordotonal organ (FCO), were determined by making intracellular recordings from motor neurons and spiking local interneurons in the central nervous system and from afferent cell bodies in the periphery. Staining the central projections of the afferent neurons with dye introduced into their axons at the receptor, and the intracellular injection of dye into motor neurons and interneurons, shows that the branches of all 3 types of neuron overlap in specific regions of neuropile. Afferents excited by a movement of the receptor apodeme that is equivalent to an imposed extension of the femorotibial joint excite flexor tibiae motor neurons and some spiking local interneurons with cell bodies at the ventral midline of the metathoracic ganglion. The opposite movement excites extensor tibiae motor neurons and a different set of spiking local interneurons. Spikes in afferents that excite flexor motor neurons evoke depolarizing potentials that follow each spike with a consistent central latency of approximately 1.5 msec. The amplitude of the depolarizing potentials is dependent upon the membrane potential of the motor neuron. This evidence points to the connection being direct and to the potentials' being EPSPs. Simultaneous recordings from certain spiking local interneurons and certain flexor motor neurons show that they receive many synaptic potentials in common and are driven in a parallel fashion by movements of the receptor apodeme. Spikes of some afferents evoke EPSPs in both neurons with the same consistency and latency. An afferent can therefore synapse directly upon a motor neuron and a spiking local interneuron. Each afferent synapses on several motor neurons and possibly upon several interneurons. In turn, each motor neuron and each interneuron receives inputs from several afferents.

Animals↗

PubMed1987

Inhibitory interactions between spiking and nonspiking local interneurons in the locust.

Simultaneous intracellular recordings were made from pairs of spiking and nonspiking local interneurons in the metathoracic ganglion of the locust to search for interactions that might underlie tactile and proprioceptive reflexes of a leg. A spike in a spiking local interneuron is followed after a consistent latency (0.6 +/- 0.12 msec, mean +/- SD) by an IPSP in a particular nonspiking interneuron. The connection appears to be direct and chemically mediated. By contrast, manipulating the membrane potential of a nonspiking interneuron by injecting current through the recording electrode has no direct effect on a spiking local interneuron. The direct interactions between pairs of these local interneurons are thus one-way. If, however, the current injected into a nonspiking interneuron is sufficient to evoke a movement by exciting motor neurons, then the spiking interneuron can be excited or inhibited by the resulting reafference. The spiking local interneurons have excitatory regions in their receptive fields formed by arrays of exteroreceptors or by proprioceptors at specific joints. The inhibitory connections mean that the postsynaptic nonspiking interneurons have corresponding inhibitory regions to their receptive fields. Several spiking local interneurons with similar receptive fields may converge onto one nonspiking interneuron. Some nonspiking interneurons, however, have larger receptive fields than an individual spiking interneuron, again indicating convergence of inputs. The specificity of the inhibitory connections preserves the spatial representation of sensory information for use in particular reflexes. For example, touching hairs on the ventral femur evokes a reflex extension of the tibia. Spiking interneurons excited by these receptors inhibit a nonspiking interneuron that would cause the opposing and therefore unwanted flexion movement. Viewed in this behavioral context, the pattern of connections between the local interneurons forms the basis of the circuitry for the local reflex adjustments of posture and locomotion.

Action Potentials↗

PubMed1986

Spiking local interneurones in the mesothoracic ganglion of the locust: homologies with metathoracic interneurones.

Two bilaterally symmetrical groups of spiking local interneurones have been characterized in the mesothoracic ganglion of the locust. The cell bodies of one group, the "midline group," lie at the ventral midline. Their primary neurites run in the ventral loop of ventral commissure II to form extensive branches in the neuropile of one-half of the ganglion. A dorso-ventral process in the perpendicular tract links two distinct fields of branches, one ventral and consisting of numerous fine branches of a uniform texture that arise from stout secondary neurites, and the other more dorsal consisting of fewer branches of a varicose appearance. Cell bodies of the second, the "anterior-lateral" group, lie close to the lateral edge of an anterior connective. Their primary neurites run in a more anterior ventral commissure and their neuropilar branches are divided into two fields by a process in a more anterior dorso-ventral tract. Within the two groups, each interneurone has its own distinctive shape that is an elaboration on these basic plans. Each interneurone also has its own characteristic physiology, being excited by a particular array of mechanoreceptors on the middle leg on the same side of the body as its neuropilar branches. The receptive fields of the interneurones, defined in this way, can be extensive and cover a particular surface of all parts of the leg, or restricted to one surface of, for example, the tarsus. These interneurones therefore bear a striking resemblance to two groups of spiking local interneurones in the adjacent segmental ganglion of the metathorax.

Action Potentials↗

PubMed1986

Processing by local interneurons of mechanosensory signals involved in a leg reflex of the locust.

At the distal end of the tibia of a locust hind leg are 2 pairs of movable spurs that can be moved by contact with external objects--as, for example, when the body sways from side to side and loads one leg unevenly or when the foot is placed on rough ground--but not by direct muscular action. Movements imposed on a spur evoke phasic bursts of spikes in the axon of a single receptor cell at its base. If the displacement is maintained, however, the response adapts within a few seconds. The afferents from these spur receptors excite particular spiking local interneurons with cell bodies at the ventral midline of the metathoracic ganglion. Each afferent spike is consistently followed at a constant latency by a depolarizing potential in one of these interneurons. The potential can evoke a spike, and its amplitude is enhanced by a hyperpolarization applied to the interneuron. The central delay to this chemically mediated EPSP, which also includes conduction time to synaptic sites, probably indicates a direct connection. Some spiking local interneurons are excited by the 2 anterior spurs but are unaffected by the 2 posterior ones, while others receive the converse pattern of inputs. The receptive fields of these interneurons also include regions on either the anterior or posterior surfaces of the tibia with excitatory inputs from hair afferents. A reliable inhibitory reflex effect on the single levator tarsi is evoked by movement of any of the 4 spurs. The inhibitory potentials are not caused directly by the sensory afferents but involve the spiking local interneurons upon which the afferents synapse. The receptive field of this motor neuron therefore results from the convergence of inputs from a few interneurons. Motor neurons of other tarsal muscles are unaffected by movement of the spurs, but those of some more proximal muscles may be excited. These reflex effects should enhance the traction of the tarsus with the ground.

Afferent Pathways↗